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Geochemical and scintillometric characterization and correlation of amethyst geode-bearing Paraná lavas from the Quaraí and Los Catalanes districts, Brazil and Uruguay
- LÉO A. HARTMANN, WILSON WILDNER, LAUREN C. DUARTE, SANDRO K. DUARTE, JULIANA PERTILLE, KARINE R. ARENA, LAURA C. MARTINS, NORBERTO LESSA DIAS
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- Journal:
- Geological Magazine / Volume 147 / Issue 6 / November 2010
- Published online by Cambridge University Press:
- 19 July 2010, pp. 954-970
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- Article
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Geochemical studies of the six lowermost lava flows of the Cretaceous Serra Geral Formation (Paraná volcanic province) in Quaraí (Brazil) and Artigas (Uruguay) were combined with flow-by-flow field studies of structures and scintillometric profiles to establish a consistent regional stratigraphic framework over at least 100 km. This greatly improves exploration capability for amethyst and agate geodes. A basalt, colada Mata Olho (Alegrete facies, Serra Geral Formation), was the first lava to flow over the ancient Botucatu desert in the region, but an andesite, colada Catalán, overstepped this basalt in many places, perhaps palaeohighs. Four basaltic andesites complete the lava stratigraphy in this formation, adding up to 300 m of lavas. The stratigraphic sequence of contrasting lava compositions is 51.0 wt% SiO2 in the first lava, followed by 57.5, 52.5, 56.0, 53.0 and finally 54.5 wt% SiO2. Overall MgO variation is between 2 and 7 wt%. All lavas in the two districts are low-Ti (<2.0 wt% TiO2) of the Gramado type. The characteristic contents of most major and trace elements (124 rock samples analysed) allow the ready identification of each lava. Contrasting rock chemistry also results in strong variation in scintillometric values (270 points measured in the field and nineteen continuous borehole profiles); from bottom to top of the stratigraphy, the cps values are 49±3.2, 123±10.3, 62±4.7, 94±4.6, ~45 and ~85. Colada Catalán has the structure of aa lava, particularly the contorted igneous banding and autobreccias in the upper and lower crusts. In some places, a 2 m thick, silicified sandstone layer lies on top of some coladas, and silicified sandstone forms breccias with volcanic rocks. Geochemistry of the six lavas indicates complex evolution, involving melting of lithospheric mantle, injection into the crust and assimilation of crust followed by fractional crystallization. This study indicates the possibility of world-class deposits of amethyst geodes on the Brazilian side of the border with Uruguay.
Contributors
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- By Jennifer Alvarez, Ananda B. Amstadter, Metin Başoğlu, David M. Benedek, Charles C. Benight, George A. Bonanno, Evelyn J. Bromet, Richard A. Bryant, Barbara Lopes Cardozo, M. L. Somchai Chakkraband, Claude Chemtob, Roman Cieslak, Lauren M. Conoscenti, Joan M. Cook, Judith Cukor, Carla Kmett Danielson, JoAnn Difede, Charles DiMaggio, Anja J.E. Dirkzwager, Cristiane S. Duarte, Jon D. Elhai, Diane L. Elmore, Yael L.E. Errera, Julian D. Ford, Carol S. Fullerton, Sandro Galea, Freya Goodhew, Neil Greenberg, Lindsay Greene, Linda Grievink, Michael J. Gruber, Sumati Gupta, Johan M. Havenaar, Alesia O. Hawkins, Clare Henn-Haase, Kimberly Eaton Hoagwood, Christina W. Hoven, Sabra S. Inslicht, Krzysztof Kaniasty, Ronald C. Kessler, Rachel Kimerling, Richard V. King, Rolf J. Kleber, Jessica Mass Levitt, Brett T. Litz, Maria Livanou, Katelyn P. Mack, Paula Madrid, Shira Maguen, Paul Maguire, Donald J. Mandell, Charles R. Marmar, Andrea R. Maxwell, Shannon E. McCaslin, Alexander C. McFarlane, Thomas J. Metzler, Summer Nelson, Yuval Neria, Elana Newman, Thomas C. Neylan, Fran H. Norris, Carol S. North, Lawrence A. Palinkas, Benjaporn Panyayong, Maria Petukhova, Betty Pfefferbaum, Marleen Radigan, Beverley Raphael, James Rodriguez, G. James Rubin, Kenneth J. Ruggiero, Ebru Şalcıoğlu, Nancy A. Sampson, Arieh Y. Shalev, Bruce Shapiro, Laura M. Stough, Prawate Tantipiwatanaskul, Warunee Thienkrua, Phebe Tucker, J. Blake Turner, Robert J. Ursano, Bellis van den Berg, Peter G. van der Velden, Frits van Griensven, Miranda Van Hooff, Edward Waldrep, Philip S. Wang, Simon Wessely, Leslie H. Wind, C. Joris Yzermans, Heidi M. Zinzow
- Edited by Yuval Neria, Columbia University, New York, Sandro Galea, University of Michigan, Ann Arbor, Fran H. Norris
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- Book:
- Mental Health and Disasters
- Published online:
- 07 May 2010
- Print publication:
- 20 July 2009, pp xi-xvi
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